I’m so sorry to read this, and am no expert, so can only offer a suggestion that maybe if your dad admits he has a problem you convince him to have his wages paid into another account he doesn’t have access to, and you transfer what he needs to spend to his own account? It may not solve the problem if he chooses to spend that money on drugs, but maybe at least it limits his spend, and alleviates the financial pressure on your Mum.
If he is in a high pressure job, and depending on the company, it could also be worth seeing if he could get a 3 or 6 month sabbatical so he can take some time away from work pressure.
He wouldn’t have to say why, but just that he needs some time for family. This is becoming quite normal in many industries as companies understand how “burnout” can affect people.
Wish you all the best x